The image coding and decoding method based on the H.26L coding method is known as a case example of the conventional video coding methods, which is described in International Standardization Working Document ITU-T SG16 VCEG-M81, “H.26L Test Model Long Term Number 7 (TML-7)” draft0 (http://standard.pictel.com/ftp/video-site/0104—Aus/VCEG-M81d0.doc).
In this H.26L video coding, many motion division patterns shown in FIG. 4A to FIG. 4G are prepared as inter prediction modes so as to enable elaborate motion compensation. This ingenuity presents such effect that it is feasible to efficiently capture motions in a case where fine motions occur inside macroblocks, and to increase prediction efficiency.
However, the method of preparing the many motion division patterns as inter prediction modes has the problem that more motion vector information needs to be transmitted with increase in the number of motion divisions, so as to result in increase of the overhead. FIG. 5 shows the volume of transmitted information necessary for each of the modes of FIG. 4A to FIG. 4G. In this FIG. 5 “mode bit” represents a code length of a variable length code for expressing each prediction mode, “minimum motion vector bit” represents the minimum information amount that can be taken as the amount of information about motion vectors to be transmitted in each macroblock unit, and “minimum requisite bit count” represents the sum of the mode bit and the minimum motion vector bit. The motion vector bit is the total bit count of two values in the horizontal and vertical directions, and the case yielding the minimum is one wherein the motion vector agrees with a motion vector predicted value in both the horizontal and vertical directions (a predicted value of a motion vector is calculated from near motion vectors according to a predetermined rule and the difference between the motion vector value and the motion vector predicted value is coded). However, that is normally a rare case, and it is necessary to transmit more information than the bit count. It is seen from this FIG. 5 that the overhead becomes very large in Mode 4 or 5 or higher.
On the other hand, in the case of ordinary images, it is believed that there exist cases wherein sufficient motion compensation can be achieved by a comprehensive way of capturing the motions by division into two segments or so, without elaborate motion detection based on segmentalization of each macroblock region into four or more segments. However, the fixed motion assignment in the 16×8 and 8×16 pixel units prepared in the H.26L coding method had the problem that it was not possible to accurately express motion divisions inside each macroblock, which was the reason why the seven inter prediction modes of FIG. 4A to FIG. 4G were prepared.
The present invention has been accomplished in order to solve the above problem and an object of the present invention is to provide a coding method, a decoding method, a coding apparatus, a decoding apparatus, an image processing system, a coding program, and a decoding program enabling coding or decoding of motion information in a picture with the overhead being small.